The welding of two molded parts, called saddle parts, to a plastic pipe is disclosed in DE-B2-2242369, wherein a heating mat surrounds the pipe without covering it. To compensate for large deviations within the pipe diameter tolerance, particularly for larger diameter pipe, sufficiently large clearances must be provided between the mat ends and between the molded parts. Thus, complete welded connections are not guaranteed about the entire pipe periphery.
Also, no welded connection is formed with such saddle parts along both clearances so that no completely sealed weld connection is possible between one or two pipes and two molded parts. Thus, such system is limited to the welding of a saddle part provided with a branch piece with a pipe.
For connecting two pipes, it is known to use welding sleeves (e.g., CH-A 396 536). Conventional welding sleeves produce weld connections which are sealed on all sides. However, since the conventional sleeves cannot be attached radially to a previously laid pipe, the sleeves cannot be used for sealing leaky pipelines or for mounting branch lines on laid pipelines.